WRT the Sony RX10-IV, yes it has deficiencies. I only use the electronic viewfinder and find the contrast too low and hummingbirds flutter rather than fly, but the spin dial exposure adjustment is nice in this viewfinder as you can see the brightness change as you spin it.. I did purchase an extra battery and a separate battery charger. I set it to 30 fps on my first outing and had half a battery charge remaining after exposing 1800 images. Now I shoot at 10 fps and the battery is not as good as in the DSLRs. The resolution of the Zeiss lens is not as good as with the Sigma and Canon lenses I have used (but likely adequate for the 20 MP on a 1-inch sensor). However, when shooting wildlife the limiting factor is the subject, not the lens. Not studio photography. All the fancy gear in the world won't help if you can't find the subject being sought. And if the subject is unpredictable and "flighty", luck may supersede many camera features.
For example, last Summer I heard a certain bird, but to get close enough for photos, I had to negotiate a steep slope of loose dirt and gravel. I knew the heavy gear would be able to take better photos, but I took the Sony instead because of the terrain. I have taken razor sharp images of very speedy Kinglets with the Sony. The silent shutter helps. It has a 600-page manual just for an English-only version. You have to study and fiddle in order to get the most out of this camera. If the next version is as big of a jump in features as this one is over the III version, I know I'll buy it and find it even more of a substitute over the big, heavy gear.
I just think that Canon has foundered in recent times. I think the mirrorless camera bodies announced along with the suggested lenses will be competing with smartphones in the future and they will lose. Google's Night Sight, for example, may be better at shooting Owls than any current DSLR. However, a mirrorless upgrade of the 7D II will not be competing with smartphones. But I'd want lenses for such a body designed for it, not restricted by adapters.